Process for production of alpha alumina bodies by sintering seeded boehmite made from alumina hydrates

ABSTRACT

Inexpensive hydrates of alumina are used for the production of dense, submicron alumina bodies by conversion of the alumina hydrates to a soluble form which is then used to produce a boehmite gel. The boehmite gel, with very small particles of alpha alumina intimately dispersed therein is dried and fired to 1300° C. to 1450° C.

BACKGROUND OF THE INVENTION

Recently it has been discovered that colloidal boehmite (alpha aluminamonohydrate) when formed into a gel or gel-like paste, and seeded withultrafine (less than 0.1 micron) alpha alumina particles, can beconverted, by relatively low temperature firing into alpha aluminabodies of extremely fine crystal size (0.5 microns and finer) andsubstantially full density. The final density can be less, if desired,when shorter firing time or lower temperature or both are employed.

The final product alumina may be in the form of shaped bodies, such astubes for filtration and the like, high desnity substrates forelectronic purposes, wear resistant or refractory parts, and abrasivegrits. The alumina may also form a matrix for bonding other refractoryabrasives, or wear resistant materials.

One problem with the commercial development and use of such products isthe relatively high cost of the suitable colloidal boehmites nowavailable.

Current production of colloidal boehmite in industry stems from two mainroutes:

1. Aluminum alkoxide (or other Al-organic) hydrolysis

2. Neutralization of sodium aluminate liquors

The first method relies on aluminum powder (of high purity). In the pastthe boehmite generated was actually a by-product of the Ziegler processfor production of linear alcohols and hence production was limited bythe alcohol market. However, it is possible to form the alkoxide andthen recycle the alcohol to eliminate this dependence on the alcoholmarket. Clearly, the economics of this process depend largely on theprice of aluminum powder, a relatively expensive commodity.

The second technique requires precipitating the boehmite (or precursor)from a solution of sodium aluminate. This results in some loss ofreagents and requires that extensive washing of the precipitate be doneto eliminate sodium salts which are generally deleterious to the endproduct application. While precipitation from sodium aluminate itself isnot a costly procedure, the loss of reagents and extensive washing(filtering) makes this process relatively expensive as well.

DETAILED DESCRIPTION OF THE INVENTION

The preferred method of preparation of colloidal boehmite for use inthis invention involves digestion of hydrated alumina such as boehmite,gibbsite, or hydrargillite with a volatile acid such as nitric. Thealumina hydrate may be in the form of commercial prepared product suchas Alcoa hydral 710, or if certain impurities can be tolerated may be inthe form of raw bauxite or other mineral source of acid soluble alumina.Caution must be exercised in using commercial alumina hydrates tominimize their sodium oxide contents; for production of hard (dense)alpha alumina, the soda content should be less than about 0.2 w/o,preferably less than 0.1. The soda content of most bauxites isnegligible but many other impurities are present, however in someapplications these can be tolerated.

The digestion of alumina hydrate which acid is a well known chemicalreaction. The product aluminum nitrate may be used in solution form, ordried (crystallized) as the hydrate Al(NO₃)₃.9H₂ O. It will be realized,of course, that any sodium present in commercial hydrate will dissolveas well. Similarly oxides of iron will dissolve along with the aluminaif bauxite is employed.

Also small amounts of other impurities will dissolve analgously (SiO2,TiO2).

The dissolution is best doen in stirred vessels at elevated temperatures(near the boiling point). Acids other than nitric may be used: sulfuricis particularly effective in the dissolution step but later processingmakes nitric the preferred acid.

Once the solution (or solid product) of aluminum nitrate is obtained,colloidal boehmite may be formed by two different routes:

(i) Neutralization with base (NH₄ OH or NH₃).

(ii) Denitration at elevated temperature followed by autoclaving onaqueous slurry in the boehmite phase field of temperature and pressure.

In method (1) it has been found that solutions of about 2 m areeffective to producing boehmite with aqueous ammonia added in excess asa 28 weight percent solution. Gaseous ammonia may also be bubbledthrough to obtain similar results. The reaction is Al(NO₃)₃ +3NH₄OH→ALOOH+3NH₄ NO₃. The aqueous ammonia is preferably added as rapidly aspossible to a warm solution of the nitrate. There is no need to warm theammonia solution as the enthalpy of neutralization serves to heat it asit is added. It is preferred that the resultant gelatinous mass becontinuously and rapidly stirred as neutralization proceeds and, thatonce additions are complete, to let the mass age and dehydrate forseveral hours (e.g. overnight) at ≧80° C. The addition of alpha aluminaseeds may take place either before, during, or after hydrolysis withoutany appreciable differences to the fired product.

After dehydration the gelatinous mass contains large amounts of ammoniumnitrate by-product. This may be eliminated by filtering, but thepreferred method is to volatilize the bulk of it by a roasting step asfollows:

    NH.sub.4 NO.sub.3 +3/20.sub.2 →heat→2NO.sub.2 +2H.sub.2 O.

The reaction is, in fact, much more complex than depicted above but isillustrated for simplicity.

It is desirable to capture the evolved NO₂ (actually NO_(X)) and convertit to nitric acid which may be recycled into the dissolution phase ofthe system. This conversion is possible and, in fact, is the basis forcommercial production of nitric acid as for example by AmericanCyanamid. In that process 3 moles of NO₂ react with one mole of water toproduce 2 moles of nitric acid and one mole of NO. Oxygen is introducedto oxidize the NO to NO₂ which is recycled in the system.

In the alternative process (ii) the solid aluminum nitrate is recoveredby evaporization and crystallization from the aluminous nitratesolution. The hydrated crystals are dried and roasted to drive off atleast enough oxides of nitrogen to bring the Al₂ O₃ to nitrate moleratio to greater than 2.5/1. The nitrate depleted material is thenautoclaved under autogenous pressure at 150° C. to 300° C. to convertthe material to micro-crystalline boehmite. While it is more difficultto achieve a crystal size below 150 Angstroms in this autoclavingprocess, the process has the advantage of not requiring the use ofammonia. For best results the boehmite crystal size should be less than150 Angstroms no matter which process (i) or (ii) is used. Mostpreferable is a size of 100 Angstroms or less.

In the following examples the term "milled water" refers to a watersuspension of submicron alpha alumina particles produced by "milling"water with alumina grinding media in a vibratory mill. A suitable millis such as that shown in U.S. Pat. No. 3,100,088. Typically the mediamay be sintered alumina cylinders 1/2" in diameter by 1/2 to 3/4" long.The interior surface of the mill is preferably lined as with rubber orplastic to avoid contamination by metal walls. Milling for 10 to 12hours is sufficient to produce a water suspension of suitable seedmaterial. The debris (alumina) in the milled water has a typical surfacearea (measured by the B.E.T. nitrogen absorption method) of 40 squaremeters per gram. This corresponds to a theoretical particle size of theorder of 0.04 microns. An alternative to the use of such milled water isthe use of the supernatant liquid when a very fine commercial alumina isallowed to settle for several days in a water suspension.

Seed material of the proper size is effective in amounts as small as0.1% by weight of alumina solids. No advantage is achieved in additionof over 5% by weight, based on the weight of the fired alumina solids.Although the optimum sizing for the seed material is not known, clearlyit must be below 0.1 microns, and probably much finer. As used in thisinvention "effective amount" of seed means that amount of seed whichresults in a density of 3.9 grams/cc or higher and a crystal size in thefired alpha alumina of less than one micron when fired at 1350° C. forfive minutes. When using "milled water" as a source of seed, the optimumamount of seed is 1% by weight of the alumina solids.

Again during the roasting operation, the oxides of nitrogen are capturedand recycled as nitric acid; alpha alumina seeds are added and thesol-gel dried and fired as in process 1. This procedure has theadvantage of not requring ammonia but has the disadvantage of requiringan autoclaving step.

Example 1. Illustrates Utility of Method (i).

75 g of reagent grade Al(No₃)₃, 9H₂ O was dissolved in 100 ml tap waterand heated to 90° C. Rapidly and with vigrous stirring 80 ml of 28 w/oNH₄ OH solution (at room temperature) was added. The temperature of thereaction mixture was about 85° C. after the NH₄ OH addition owing to theenthalpy of reactions. To this gelatinous slurry was added 3 grams of"milled water" containing 6% of alpha alumina seeds. The whole was leftat 85° C. on a hot plate overnight to give a moist gel. This was placedunder heat lamps to vaporize the bulk of NH₄ NO₃ to obtain hard drylumps of white gel. These lumps were crused to abrasive grits through a28 mesh screen and on a 44 mesh screen. The X-ray diffraction pattern ofthese glassy grains revealed boehmite of extremely fine crystalline size(35-50 A).

The grains were plunged into a tube furnace at 1390° C. for fourminutes, cooled and examined. They were translucent-white granules witha hardness of 20 GPa. Empirically these grains looked equivalent to orbetter than any produced from commercial boehmite gels.

Example 2. Illustrates Utility of Method (ii).

A beaker containing about 300 g of Al(NO₃)₃.9H₂ O was heated in a mufflefurnace at 270° C. for 16 hours. The resulting fluffy-caked mass had aloss on ignition of 18% calculating to an Al2O3/NO3 ratio of about 2.8to 1 (assuming nitrate as the only volatile). The X-ray diffraction ofthis material showed it to be completely amorphous.

25 Grams of this was pulverized to -80 mesh and placed in a stainlesssteel 1 liter autoclave with 250 ml of tap water. The autoclave washeated to 160° C. in 1/2 hour, held at 160° C. for 21/2 hours and cooledin about 15 minutes. This yielded a translucent sol of low viscositywhich was evaporated at 90° C. to a dried translucent cake which X-raydiffraction revealed to be boehmite with ultimate crystallite diameter60-110 A, FIG. 5. This material was crushed as in example 1 and fired at1370° C. for 4 minutes to give an opaque-translucent body which had ahardness of 18 GPa. Such grains would have excellent utility as abrasivematerials.

When the roasting of the Al(NO₃)₃.9H₂ O was carried out for only 8 hoursa cake which analysed as Al₂ O₃ HNO₃ was obtained which yielded a whitegel on autoclaving but did not sinter well tending to crumble and becomechalky.

Example 3. Illustrates method 1 with Bauxite starting material.

80 Grams of abrasive grade bauxite which had been Sweco milled for 22hours was mixed with 200 ml tap water and 160 ml 70% nitric acid, heatedto 110° C. and left to digest for 2 hours at this temperature. The hotslurry was then filtered with a Bucher funnel using glass fibre filterpaper and the residue washed with 3×50 aliquats of tap water. Theresidue was dried and weighed 12.4 g indicating that about 85% of thebauxite dissolved.

200 ml of the solution after filtering was heated to 90° C. and 160 mlof 28% NH₄ OH added rapidly. This gelatinous slurry was vigourouslystirred and 4 ml of 6.25% "milled" water also stirred in with anadditional 50 ml water. This was dried at 90° C. over 24 hours andfurther dehydrated/denitrated at 200° C. under hot lights for 40 hours.This material X-rayed as boehmite of 100 Angstroms crystallite size. Thegel was crushed to grains -28 +44 mesh and fired at 1380° C. for 4minutes to give a light brown glassy grit which had average hardness of16.5 GPa; also some grits were observed as high as 18.5 GPa.

The processes described herein permit production of alpha alumina at alower cost and smaller ultimate crystalline sizes than commerciallyavailable at present. The process also enables control at each step tobe exercised thereby avoiding present commercial product variability.

What is claimed is:
 1. A method of making articles containing submicronalpha alumina in at least one phase, comprising (1) dissolving aluminahydrate in nitric acid, (2) converting the thus formed aluminum salt toboehmite having a crystal size below 100×10⁻¹⁰ meters by nuetralizationwith ammonia, converting said boehmite to a gel by acidification, andfiring said gel in the presence of at least 0.5% by weight of alphaalumina submicron seed particles to a temperature less than 1450° C. fora time such that the thus formed alpha alumina phase has an averagecrystal size of less than 0.5×10⁻⁶ meters.
 2. A method as in claim 1 inwhich the aqueous aluminous salt solution formed in step (1) isneutralized with ammonium hydroxide to form boehmite having a crystalsize below 100×10⁻¹⁰ meters.
 3. A method as in claim 1 in which thealuminum salt is a nitrate salt and is dried and fired to increase theAl₂ O₃ lNO₃ ratio to at least 2.5 to 1 and then autoclaved at 150° to300° C. to produce boehmite having a crystal size of less than 150×10⁻¹⁰meters.
 4. A method as in claim 1 in which the alumina hydrate is in theform of naturally occuring ore.
 5. A method as in claim 4 in which theore is bauxite.